A stem cell is a generic name for an undifferentiated type of cell found in tissues of embryos, fetuses and adults, which is characterized by the ability to differentiate into a diverse range of specialized cell types. In various classifications of stem cells, pluripotent stem cells refer to cells which may differentiate into cells derived from any of the three germ layers constituting the living body.
In the classification, stem cells may be classified according to an anatomical region, the function of cells, the type of antigen expressed on the cell surface, a transcription factor, a protein generated by cells and the type of specific cells which may be generated by the stem cells.
One of the most frequently used criteria is a source from which stem cells are derived. The stem cells may be classified into embryonic stem cells (ES cells) which are isolated from an embryo, and adult stem cells that are isolated from an adult.
According to another frequently used classification of the number of types of cells differentiated from a stem cell, stem cells may be classified into pluripotent stem cells, multipotent stem cells, and unipotent stem cells. In general, embryonic stem cells are representative of pluripotent stem cells, and adult stem cells are separated into multipotent stem cells and unipotent stem cells.
The inner cell mass of a blastocyst which is formed in the early development of mammals after fertilization is a part which subsequently forms the embryo, and embryonic stem cells formed from the inner cell mass may be referred to as pluripotent stem cells having the potential to differentiate into cells of all tissues constituting the individual. That is, the embryonic stem cells are undifferentiated cells which are able to proliferate without limitation, differentiate into all types of cells, also form germ cells in contrast to adult stem cells, and thus may be inherited by the next generation.
However, the preparation of these embryonic stem cells, that is, pluripotent stem cells results in the destruction of the human embryo, which raises religious and ethical issues. Further, since embryonic stem cells are derived from limited embryos, there is a lack of immune compatibility between each individual, and thus there may be transplant rejection in the development of embryonic stem cells as cell therapeutics. In order to address these issues, there have been various attempts to artificially produce pluripotent stem cells mimicking embryonic stem cells by using adult-derived cells.
Examples of the representative methods include somatic cell nuclear transfer (SCNT), fusion with ES cell, reprogramming by defined factor, etc. SCNT requires a large amount of oocytes due to the very low efficiency thereof. Fusion with ES cells has a problem in stability because the pluripotent cells induced thereby contain two additional pairs of genes. Reprogramming by defined factors, which is the most recent technique, employs a virus containing an oncogene, and thus may cause tumorigenicity.
In order to ensure customized pluripotent stem cells for the development of cell therapeutics, there is a need for the development of a method of producing pluripotent stem cells which is safe and may address the ethical issues. According to this need, the inventors of the present invention have developed dedifferentiated stem cells using an extract of induced pluripotent stem cells (iPS) derived from animals, but there are some restrictions thereof.
First, about 20 mg or more of an iPS extract is required to perform the above-described method. Accordingly, the dedifferentiation induction in which an extract is isolated from human dedifferentiated stem cells, which requires much cost and labor, has not yet succeeded. For example, those skilled in the art have to spend 3 months or more to prepare human dedifferentiated stem cells to obtain 20 mg of an extract, requiring a high cost.
Second, animal stem cells or dedifferentiated stem cells corresponding thereto are not completely crushed and are remained in the process of separating the extract, when somatic cells to be induced to dedifferentiate are treated with the extract, it is difficult to distinguish cells induced to dedifferentiate from dedifferentiated stem cells remaining in the extract, and thus it may not be immediately determined whether the experiment succeeded or not, and may be determined after analyzing the genomic DNA thereof. Accordingly, a lot of time and cost may be wasted.
Third, since there have been rare cases of human dedifferentiated stem cells produced using proteins, an extract of human dedifferentiated stem cells produced using a virus is used. However, this extract may contain cancer-causing substances induced by the virus, which may be an obstacle to clinical application.